1. Field of the Invention
The present invention relates to polyester-based composite films having good gas-barrier properties, and, more especially, to polyester/PVA such barrier films.
2. Description of the Prior Art
Polyester films, especially of polyethylene terephthalete, are today widely employed in the packaging industry, because of their many advantages such as their mechanical properties, their transparency, their nontoxicity, their odorlessness and their tastelessness.
However, their gas-barrier properties may limit their use in applications requiring a high protection of the packaged products against the action of external gases, in particular against the action of atmospheric oxygen or, conversely, may not allow the gas composition inside the packages to be maintained constant.
In order to overcome this disadvantage, GB-A-1,126,952 describes the deposition of a solution of polyvinyl alcohol ("PVA") onto a polymer film to create a polyvinyl alcohol layer, since this latter polymer has good gas-barrier properties. To provide good adhesion of the polyvinyl alcohol layer to films of cellulose acetate, polycarbonate or polyethylene terephthalete, an intermediate layer of an adhesive made of polyurethane is deposited between the base film and the polyvinyl alcohol layer.
EP-A-0,254,468 also describes a composite film comprising a base film of a synthetic thermoplastic polymer such as a polyamide, polyethylene, polypropylene or polyester, having two coatings on the same single face surface to the base film; the first coating is adjacent to the base film and comprises a urethane primer in a solvent, which in the dry state permits a dispersion of polyvinyl alcohol in aqueous solution to wet the primer, and the second coating is deposited over the dried and free face surface of the first coating and includes a polyvinyl alcohol-based material employed as a gas barrier.
In the composite films of the prior art the polyvinyl alcohol coating is typically separated from the base film by an adhesive layer comprising a polyurethane. Moreover, the polyvinyl alcohol layer is generally thick (for example ranging from 0.15 to 2 micrometers, per the claims of the aforesaid EP-A-0,254,468), and this complicates the recyclability of the composite films.